Transferring a licensed user from one Pro account to another Pro account

I am the owner of two Workplace Pro accounts for our small business. I want to transfer one of the licensed users from account A to account B. The user has no unique settings, data or recordings that need to be brought over. He will use the same business email address. I just want him to be a user in account B, not account A. 

Do I delete the user from account A and then assign him a license from account B? Or can I transfer his license from account A, thereby avoiding purchasing a license in account B?